1. Field of the Invention
The invention relates to a mobile mixer, preferably having counterrotational emptying, for building materials, in particular concrete, according to the preamble of claim 1.
2. Description of the Prior Art
The mobile mixer according to the invention transports a building material charge in the mixing drum and thoroughly mixes this charge during transport by the rotation of the mixing drum. Apart from consisting of concrete, the building material can also consist of mortar, with lime mortar, cement mortar and fire-clay mortar also being considered in addition to wall mortar and plaster mortar. Considered generally, these building materials consist of a special dry mixture which, apart from the aggregates or sand and the admixtures, usually contain hydraulic, but also occasionally non-hydraulic cements, and of water. As soon as the water comes together with the cement, a chemical process starts in most cement types, which chemical process proceeds all the more quickly the higher the ambient temperature, with it being necessary in this process to take into account the released heat of hydration as an additional heat source. The mobile mixer according to the invention enables the mixing water to be added during the journey or at the application site of the building material on account of its intentionally mixing effect, which circulates the building material charge, the mixing water being added at a point in time which is more or less shortly before the building material is used at the building site, so that the mobile mixer can transport the dry mixture and can thoroughly mix the latter with the mixing water.
The design of the mixing drum in which the mobile mixer according to the invention transports the building material, because of the firmly arranged, spiral elevators on the inner wall of the mixing drum, ensures a simple construction of this device, which, depending on the direction of rotation of the mixing drum, draws the building material inwardly about its center axis or delivers it outwardly through the mixing drum opening opposite the closed mixing drum base.
The invention relates inter alia to mixing drums which are mounted at an inclined angle on, for the most part, road vehicles, designed as trucks or truck platforms, in such a way that their opening through which the building material is fed or delivered is located at the top and their closed base is located at the bottom. According to the invention, if the building material has to be transported through narrow areas, as occur, for example, in tunnel construction, the mixing drum is also constructed with a horizontal arrangement, for example on a track bogie of the mixing drum axis. In this case, the shell of the mixing drum can have a number of manholes arranged next to one another between the mixing drum base, which are closed with a removable lid, and the mixing drum opening, which number of manholes depends on the length and the mixing drum diameter, through which manholes the base is accessed when the lid is opened and the building material charge is put in. In these mobile mixers, the opening of the mixing drum is often used to receive the building material from a mixing drum connected on the input side and to have it run through a mixing drum connected on the output side.
In known mobile mixers of the type described, a helical elevator usually made of flat bar steel, is provided above the drum base. It exerts a driving effect on the building material, which driving effect can lead to considerable compression in the building material charge core surrounded by the elevators, which building material charge has been filled into the mixing drum for transport.
In slightly moist concrete in particular, this stops the mixing action, which leads to considerable deterioration of the building material.
For this reason, it is also known (German Offenlegungsschrif No. 2,949,026), to mount in the drum center a pipe which rotates with the drum center and in which a spiral is fixed which rotates in the opposite direction to the outer mixing spiral. The pipe ends above the drum base and beneath the drum opening. However, the counterflow generation intended therewith in the core of filling in the drum, which counterflow loosens the accumulation of building material on the drum base to restart the mixing action, does not occur. On the contrary, the pipe prevents mixed material, carried upward by the outer spiral in the course of a drum rotation, from being able to freely fall far enough downward. Therefore, the intended free-fall mixing does not take place or takes place only inadequately. Moreover, the narrow cross section of the passage of the pipe restricts the quantity of mixed material to such a large extent that such mixers are unimportant in practice.
If the procedure is then adopted of adding most of the mixing water or its total quantity to the building material mixture before the charge to be transported is filled in order to improve the mixability compared with the dry mixture, no substantial improvement is achieved. However, the transport of such a largely finished building material has the further disadvantage that it threatens to freeze at low ambient temperatures and experiences premature hardening at high ambient temperatures even during transport and during any waiting time on the building site if expensive counter-measures are not taken to alleviate or avoid completely the deterioration consequently caused in the building material.
Since the movements of the building material charge in the mixing drum are also inadequate, because of the described effects of the spiral elevations and the short counterrotating spiral pieces in the space between the elevators, the above mentioned building material deterioration occurs even at favorable outside temperatures in spite of a rotating mixing drum during prolonged transport and waiting times. In may types of mobile mixers, however, in particular in the types of mobile mixers described above which are provided for tunnel construction, even the rotation of the mixing drum causes considerable difficulties, since, inter alia, compressed-air drives are provided for tunnel construction for safety reasons in mobile mixers for this purpose.
In mobile mixers having the mixing drum arrangement mentioned which is inclined at an angle, as are provided in most of the highway transport vehicles, it is also known to provide a positive-mixing device to avoid the above mentioned difficulties, which positive-mixing device is provided with a drive which is separated from the mixing drum drive and is constructed on the outside in front of the mixing drum base. The positive-mixing device itself consists of a short shaft which is arranged on the mixing drum axis and on which are fixed several mixing vanes. These act beneath the filling line of the mixing drum, which filling line, for reasons of economy, in order to exhaust the capacity of the mixing drum, runs from the lower edge of the discharge opening, for example at an angle of 25 degrees, up to the upper inner wall, on to the lower part of the charge, thus, tightly filling the mixing drum above the mixing drum base. These mixing blades are intended to produce a radial and axial flow of the building material into the compressed building material core described. Such a device does in fact improve the mixing action and consequently also enables the dry building material to be transported and the mixing water to be added outside the mixing plants from which the building material is delivered to the mobile mixers.
On the other hand, mobile mixers made in this way prove to be exceptionally complex designs which, therefore, either cannot be applied at all to certain mobile mixers, such as, for example, the mobile mixers described for tunnel construction, or create numerous sources of breakdown because of their complicated construction and accordingly are difficult to maintain. In addition, the mixing action is also unsatisfactory. Since the building material in question, in particular if it is concrete, contains coarse aggregate materials to a large extent more or less, the tools of the positive mixer must be protected from possible damage caused by jammed constituents of the building material mixture. This can only be done if an appropriately large intermediate space is maintained between the positive-mixing device and the elevations of the drum shell, in which intermediate space the coarse constituents can turn aside, but in which on the other hand, positive mixing action is not achieved either.